Process for the manufacture of silver halide emulsions by the flocculation method



United States Patent ()fliice 3,360,373 Patented Dec. 26, 1967 ABSTRACT OF THE DISCLOSURE Photographic silver halide emulsions are prepared by the flocculation method which consists in precipitating the gelatine silver halide emulsion With a flocculating agent, separating it from the supernatant salt solution and then again dispersing it in a gelatine solution. The flocculating agent is added at a pH .of 3 to 5 to the gelatine silver halide emulsions and consists of a linear additive copolymer prepared mainly from N-vinylethylene urea, N-vinyloxazolidone or N-vinylpyrrolidone and a water soluble salt of acrylic or methacrylic acid. to 70 mol percent of the vinyl compound and 30 to 85 mol percent of the acrylic or methacrylic acid salt are'used in the copolymer preparation.

The present invention provides a process for the manufacture of photographic silver halide emulsions in which the extraneous soluble salts are removed by flocculation of the silver halide.

In the preparation of photographic silver halide emulsions, a water-soluble silver salt, usually silver nitrate, is reacted with a Water-soluble halide in an aqueous gelatine solution. The gelatine/ silver halide emulsion thus formed contains Water-soluble salts which are byproducts of the double decomposition, and these salts have to be removed. The usual method of removing the said salts consists in solidifying the emulsion by cooling, comminuting it, and then washing out the salts with cold water.

Another method, known as the flocculation method, consists in precipitating the silver halide emulsion, separating it from the supernatant salt solution, and then again dispersing it in a gelatine solution or in some other suitable layer colloid.

The flocculation method otters a number of advantages; in particular, it makes possible the preparation of concentrated silver halide emulsions. Various methods of flocculating silver halide emulsions have been proposed, for example, flocculation with an inorganic salt, for example, sodium sulfate. However, a large amount of inorganic salt is required, and has to be removed by washing the precipitate. Organic solvents can also be used for bringing about flocculation but this method is expensive and the precipitates obtained are highly voluminous and still contain a large amount of Water. It has also been proposed to use the salts of heavy metals as flocculating agents but these, are disadvantageous in that they harden the gelatine and the precipitates are very diflicult to re.- disperse. It has also been suggested that anion-active substances such as soaps be used, but in this process residual soap adheres tenaciously to the coagulate and tends to cause cloudiness in the emulsion.

Other methods are based on the use of gelatine derivatives, however, the reactive amino groups of the gelatine undergo change, thus causing a change in the photographic and colloidochemical properties .of the gelatine.

This invention is based on the observation that polymers of certain heterocyclic compounds are very suitable as flocculating agents. Accordingly, the present invention provides a process for the preparation of photographic silver halide emulsions by the flocculation method. wherein there are used as flocculating agents water-soluble, linear additive copolymers which consist mainly of polymer units of (l) 15 to mol percent of a ring saturated 2-oxo-N- vinyl heterocyclic compound, and

(2) 30 to mol percent of a water-soluble salt of acrylic acid or methacrylic acid.

Suitable 2-oxo-N-vinyl heterocyclic compounds are hetero cyclics of an aliphatic character having a sixor advantageously five-membered saturated ring. One of the ring members is a nitrogen atom carrying a vinyl group in a-position to this a carbonyl group. The other atoms forming the ring are carbon, nitrogen or oxygen, and the carbon and nitrogen atoms may also carry a lower alkyl radical. The 2-oxo-N-vinyl heterocyclic compounds used in the process of the invention advantageously correspond to the formula X CH O=C CH2 H( 3=CHz in which X represents a -CH -group or an -NH-group. As examples of N-vinyl heretocyclic compounds there may be mentioned pyrrolidone, piperidone, pyrazolidone, ethylene, urea, trimethylene urea and oxazolidone compounds.

Specially suitable are, for example, N-vinyl-oxazolidone, but above all the compounds of the Formula 1, that is to say, N-vinyl-N:N'-ethylene urea and especially N-vinylpyrrolidone. In accordance with the invention, copolymers are used that contain 15 to 70 mol percent, advantageously .25 to 50 mol percent of the vinyl heterocyclic compound.

As comonomeric compounds there are used 30 to 85 mol percent, advantageously 50 to 75 mol percent, of a Water-soluble salt of acrylic acid or methacrylic acid, for example, a calcium, ammonium or an alkali metal salt such as a sodium or potassium salt, especially sodium acrylate. The copolymers are made up primarily of structural units of the aboye-mentioned composition (1) and (2); however, they may also contain a small amount, for example, up to 10% of the copolymer, of other kinds of polymer units, for example, vinyl esters of aliphatic alcohols.

The manufacture of copolymers of the kind used in the present invention is known. By known methods polymerization is carried out in an aqueous medium at 40 to C. and with the aid of a water-soluble, freeradical forming catalyst that is not harmful to photographic properties, for example, potassium persulfate.

The compounds of the composition given above can be used as flocculating agents in the. customary manner. These materials can be exceedingly effective and depending on their constitution, small amounts produce very good flocculation at pH values between 3 and S, preferably 4.0 and 4.5, at a moderately raised temperature.

In general, flocculation is advantageously brought about by adding an aqueous solution of the flocculating agent to the emulsion, and then adjusting the emulsion to the requisite pH value. It is also possible first to adjust the pH value and then add the flocculating agent. In general, only a small amount of flocculating agent is required. The amount used also depends on the gelatine content,

the salt concentration and the pH value. The amount required for a specific case can easily be determined by preliminary tests. The silver halide precipitates along with the gelatine in the form of fine granules and settles very quickly. The precipitates contain only a very small amount of water so that a further washing is generally not necessary. Should it appear necessary, however, the precipitates can be washed once or several times with cold water, the pH being adjusted to a suitable value, if necessary. The most suitable pH value for flocculation and the requisite amount of flocculating agent depend not only on the flocculating agent itself, but also on the gelatine content of the emulsion, the salt content, and other factors. The temperature at which flocculation is carried out has no great influence; in general, it is advantageously between 30 and 50 C.

Photographic silver halide emulsions of all kinds can be prepared by the process of the invention, especially color materials that contain either dyestuff components for color development or image dyestuffs for the silver dyestufl bleaching process.

The flocculating agents to be used in the process of the present invention have an important advantage in that the small amount of flocculating agent present in the coagulate has a stabilizing influence on the emulsion so that emulsions of maximum sensitivity and free from turbidity can be obtained. Moreover, the small amount of fiocculating agent acts as a plasticizer, which means that elastic layers of high break resistance, for example, as required for positive cinema film, can be obtained. By adjusting the pH to a value between 6.5 and 7.5, the coagulates can easily be redispersed in water or gelatine solution at a temperature between 35 and 50 C. Other colloids can also be used for redispersion.

The following example illustrates the invention:

Example A solution of 50 grams of silver nitrate in 450 milliliters of water is added to a solution of grams of gelatine, 42 grams of potassium bromide and 2.2 grams of potassium iodide in 450 milliliters of water at 65 C. in the course of 20 minutes. The mixture is allowed to mature for minutes at 65 C., and then cooled to 35 C. 20 milliliters of a 10% aqueous solution of a copolymer comprising 25 mol percent of N-vinylpyrrolidone and 75 mol percent of sodium acrylate are then added, and the pH value is adjusted to 4.0 by the addition of l N sulfuric acid. The silver halide precipitates along with the gelatine and the polymer and settles out very quickly. The supernatant clear liquid is decanted and the granular precipitate washed with 1 liter of cold water to which sulfuric acid has been added in an amount sufficient to adjust the pH value to 4.0. A solution of 70 grams of gelatine in 800 milliliters of water is then added and the pH value adjusted to 7.0 by the addition of 0.1 N sodium hydroxide solution. The whole is stirred for 10 to minutes at 40 C. until the silver halide is redispersed, and then allowed to mature to its optimum sensitivity in the customary manner. An emulsion is obtained which exhibits a moderate to high degree of sensitivity, depending on the kind of gelatine used.

Similar results are obtained when one of the following aqueous copolymer solutions is added to the nitrate-containing silver halide emulsion instead of milliliters of the 10% solution of the copolymer comprising mol percent of N-vinylpyrrolidone and 75 mol percent of sodium acrylate:

(a) milliliters of a 10% solution of a copolymer comprising 50 mol percent of N-vinylpyrrolidone and 50 mol percent of sodium acrylate.

(b) milliliters of a 10% solution of a copolymer comprising 66 mol percent of N-vinylpyrrolidone and 34 mol percent of sodium acrylate.

(c) 30 milliliters of a 10% solution of a copolymer comprising 70 mol percent of N-vinylpyrrolidone and 30 mol percent of calcium acrylate.

(d) 30 milliliters of a 10% solution of a copolymer comprising 25 mol percent of N-vinylpyrrolidone, 70 mol percent of sodium acrylate and 5 mol percent of vinyl acetate.

(e) 60 milliliters of a 10% solution of a copolymer comprising 66 mol percent of N-vinylpyrrolidone and 34 mol percent of sodium methacrylate.

(f) 30 milliliters of a 10% solution of a copolymer comprising 15 mol percent of N-vinylpyrrolidone and 85 mol percent of ammonium methacrylate.

(g) 60 milliliters of a 10% solution of a copolymer comprising 40 mol percent of N-vinylpyrrolidone and 60 mol percent of potassium methacrylate.

(h) 20 milliliters of a 10% solution of a copolymer comprising 25 mol percent of N-vinylethylene urea and mol percent of sodium acrylate.

(i) 20 milliliters of a 10% solution of a copolymer comprising 25 mol percent of N-vinyloxazolidone and 75 mol percent of sodium acrylate.

What is claimed is:

'1. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists mainly of polymer units of (1) 15 to 70' mol percent of a member selected from the group consisting of N-vinylethylene urea, N- vinyloxazolidone and N-vinylpyrrolidone of the general formula I 1 0:0 CH2 H=CH1 in which Y represents a member selected from the group consisting of a -CH -NH- and O radical and (2) 30 to mol percent of a water-soluble salt of an acid of the formula wherein n is a whole number of at the most 2.

2. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists mainly of polymer units of (1) 15 to 70 mol percent of N-vinylethylene urea, and

(2) 30 to 85 percent of a water-solube salt of an acid of the formula C 0 OH H2C=C wherein n is a whole number of at the most 2. 4. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists mainly of polymer units of (1) to 70 mol percent of N-vinylpyr-rolidone, and (2) 30 to 85 percent of a water-soluble salt of an acid of the formula wherein n is a whole number of at the most 2.

5. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of mol percent of N-vinylethylene urea and 75 mol percent of sodium acrylate.

6. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a lineary additive copolymer which consists of polymer units of 25 mol percent of N-vinyloxazolidone and 75 mol percent of sodium acrylate.

7. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of 25 mol percent of N-vinylpyrrolidone and 75 mol percent of sodium acrylate.

8. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of 50 mol percent of N-vinylpyrrolidone and 50 mol percent of sodium acrylate.

9. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of 66 mol percent of N-vinylpyrrolidone and 34 mol percent of sodium acrylate.

10. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of mol percent of N-vinylpyrrolidone and 30 mol percent of calcium acrylate.

11. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of '25 mol percent of N-vinylpyrrolidone, 70 mole percent of sodium acrylate and 5 mol percent of vinyl acetate.

12. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of 66 mol percent of N-vinylpyrrolidone and 34 mol percent of sodium methacrylate.

13. A process for the manufacture of a photographic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of -15 mol percent of N-vinylpyrrol-idone and mol percent of ammonium methacrylate.

14. A process for the manufacture of a photograhpic silver halide emulsion by the flocculation method, which comprises adding to the gelatine silver halide emulsion at a pH of 3 to 5 as a flocculating agent a linear additive copolymer which consists of polymer units of 40 mol percent of N-vinylpyrrolidone and 60 mol percent of potassium methacrylate.

References Cited UNITED STATES PATENTS 2,592,107 4/1952 Azorlosa 260-785 FOREIGN PATENTS 889,760 2/1962 Great Britain.

NORMAN G. TORCHIN, Primary Examiner. R. H. SMITH, Assistant Examiner. 

1. A PROCESS FOR THE MANUFACTURE OF A PHOTOGRAPHIC SILVER HALIDE EMULSION BY THE FLOCCULATION METHOD, WHICH COMPRISES ADDING TO THE GELATINE SILVER HALIDE EMULSION AT A PH OF 3 TO 5 AS A FLOCCULATING AGENT A LINEAR ADDITIVE COPOLYMER WHICH CONSISTS MAINLY OF POLYMER UNITS OF (1) 15 TO 70 MOL PERCENT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF N-VINYLETHYLENE UREA, NVINYLOXAZOLIDONE AND N-VINYLPYRROLIDONE OF THE GENERAL FORMULA 